Lightning. lightning: an electrical discharge in the atmosphere. Electricity. Lightning, Thunder and Tornados PHYS 189. Electricity.

Transcription

1 , lightning: an electrical discharge in the atmosphere (), week 12 1 / 40

2 , lightning: an electrical discharge in the atmosphere cloud-to-cloud (80%) cloud-to-ground (20%) (), week 12 1 / 40

3 Fundamentals of electricity Electrostatics, electrical charges are either positive or negative charge is measured in Coulombs it takes electrons to have one Coulomb of charge an electrical force exists whenever there is an unequal amount of positive and negative charges an electrical force is created when charges are transferred from one material to another electrical charge is conserved (it is neither created or destroyed) (), week 12 2 / 40

4 Properties of electricity, q is the symbol for charge current: measure of charge flow (I = q/ t) voltage: potential difference between a negatively charged object and a positively charges one (like terminals on a battery); also the electric potential (), week 12 3 / 40

5 Voltage=electric potential electric potential energy, electrical energy and electrical potential ( electric potential energy is equal to the work done to move charges electric potential is the electric potential energy per charge! voltage is the same as electric potential electric potential is not the same as electric potential energy! electric potential exists when there is charge separation (), week 12 4 / 40

6 Clicker question, You have a 1.5 V AA battery and a 1.5 V D cell battery. Which one does more work for each Coulomb worth of charge that moves from the negative to the positive terminal? A. AA B. D C. they perform the same amount of work per Coulomb D. not enough information given (), week 12 5 / 40

7 Clicker question, You have a 1.5 V AA battery and a 1.5 V D cell battery. Which one has more Coulombs worth of charge stored in the cell? A. AA B. D C. they have the same amount charge D. not enough information given (), week 12 6 / 40

8 Clicker question, You have a 1.5 V battery and a 12 V battery. Which one does more work for each Coulomb worth of charge that moves from the negative to the positive terminal? A. 1.5 V B. 12 V C. they perform the same amount work per Coulomb D. not enough information given (), week 12 7 / 40

9 Clicker question, You have a 1.5 V battery and a 12 V battery. Which one has more Coulombs worth of charge stored in the cell? A. 1.5 V B. 12 V C. they have the same amount of charge stored in the cell D. not enough information given (), week 12 8 / 40

10 Charge separation in the atmosphere, Ionosphere Earth (), week 12 9 / 40

11 Charge separation in the atmosphere An electric field results from the charge difference between atmosphere and the surface of the earth (fair weather electric field) Ionosphere, Earth (), week 12 9 / 40

12 Proposed charging mechanism in thunderstorms Electric field polarizes graupel particles, Saunders 2008 (), week / 40

13 Proposed charging mechanism in thunderstorms Electric field polarizes graupel particles smaller ice particles collide and strip off the positive charge ice particle travels to the top of the cloud, Saunders 2008 (), week / 40

14 Proposed charging mechanism in thunderstorms, Saunders 2008 (), week / 40

15 Charge separation in cloud, (), week / 40

16 Clicker question, Why don t charges just flow continuously through the air and neutralize the atmosphere? A. the negatively charged surface repels the negative charges at the cloud base B. air is a good electrical insulator C. it does D. nobody knows (), week / 40

17 via a stepped leader, (), week / 40

18 : cloud to ground, 1. charge separation within the cloud enhances the electric field between the earth and the atmosphere 2. electric field begins to break down air between 3. stepped leader of negative charges forms from cloud toward ground 4. positive streamer of positive charges form from ground to cloud 5. stepped leader and positive streamer meet, forming a conduit for charge transfer between cloud and ground (), week / 40

19 more info, Langmuir Lab at NMT (), week / 40

20 Clicker question, A. Cloud to ground lightning is always initiated with negative charges in the stepped leader moving toward ground. B. Discharges in the electric field can happen without lightning going all the way to the ground. C. A bolt from the blue is a figure of speech, not an actual phenomenon. D. Sprites and blue jets are creatures in faerie tales, not electrical phenomena. (), week / 40

21 Thunder, electric current traveling in the lightning path heats the air until it explodes the shockwave from the explosion is a compression wave, or sound wave, that we call thunder (), week / 40

22 safety, Go inside (buildings or vehicles) avoid tall, high or exposed areas avoid open spaces avoid water NOAA safety (), week / 40

23 Clicker question, Air mass thunderstorms A. are thunderstorms which occupy an entire air mass B. are rare, but extremely destructive C. only last 10s of minutes because they create downdrafts which cut off their moisture supply D. occur near frontal boundaries (), week / 40

24 Air mass thunderstorms, air mass thunderstorms: consist of a number of individual convective updrafts (cells) The life cycle of an air mass thunderstorm: 1. cumulus stage 2. mature 3. dissipative (), week / 40

25 Air mass thunderstorm life cycle, air is uplifted, creating initial clouds which premoisten the environment (), week / 40

26 Cumulus stage, During the cumulus stage, temperature decreases with height at the sat d adiabatic lapse rate some of the cloud extends above the freezing level ice crystals form and grow by the Bergeron process the sky darkens below the thickening cloud & precipitation begins to fall (), week / 40

27 Air mass thunderstorm life cycle, air is uplifted, creating initial clouds which premoisten the environment heavy rain or graupel particles drag air toward the surface creating downdrafts in the areas of most intense precipitation (), week / 40

28 Mature stage During the mature stage, precipitation, lightning and thunder are most intense the top of the cloud extend to stable layers of the atmosphere which prevent further uplift strong winds at top layers push ice particles to create the classic thunderstorm anvil updrafts dominate the interior of the cloud, downdrafts occur just outside the cloud boundary (perfect conditions for turbulence which entrain dry air into cloud), (), week / 40

29 Air mass thunderstorm life cycle, air is uplifted, creating initial clouds which premoisten the environment downdrafts eventually occupy the entire cloud base, cutting off the supply of moisture heavy rain or graupel particles drag air toward the surface creating downdrafts in the areas of most intense precipitation (), week / 40

30 Dissipative stage, During the dissipative stage, supply of moisture is cutoff by downdrafts associated with heavy precipitation precipitation diminishes, sky clears the cloud boundary blurs and becomes fuzzy ; this is associated with glaciation (), week / 40

31 Clicker question, A. The glaciated parts of the cloud contains a mixture of ice crystals and supercooled water droplets. B. Only about 20% of the condensed moisture in a cloud falls as precipitation. C. Few lightning events are associated with air mass thunderstorms. D. Air mass thunderstorms only support one convective cell at a time. (), week / 40

32 Severe thunderstorms: definition, severe thunderstorms have wind speeds that exceed 93 km/hr (58 mph) hailstones larger than 1.9 cm in diameter or they spawn tornados (), week / 40

33 Severe thunderstorms: characteristics In severe thunderstorms downdrafts reinforce convective updrafts and thus intensify the storm are mesoscale phenomena (it has a size ranging from 10 s to 100 s of km) can involve a cooperation of convective cells which cluster; these are called mesoscale convective systems (MCSs) MCSs can last from 12 hours to several days can arise from supercells, which is made of a single updraft region (rather than a collection of smaller cooperating cells), (), week / 40

34 How do downdrafts reinforce updrafts in MCSs?, (), week / 40

35 How do downdrafts reinforce updrafts in MCSs? strong downdrafts associated with precipitation, (), week / 40

36 How do downdrafts reinforce updrafts in MCSs? strong downdrafts associated with precipitation generate outflow that hits the surrounding warmer air, (), week / 40

37 How do downdrafts reinforce updrafts in MCSs?, strong downdrafts associated with precipitation generate outflow that hits the surrounding warmer air causing the warm air to rise and thus reinforce updraft for another convective cell (), week / 40

38 Generation of new cells in a mesoscale convective complex (MCC), (), week / 40

39 Squall lines, linear band of individual storm cells ( 500 km long) usually form in warm sector of midlatitude cyclone in front of the cold front looking for squall lines (), week / 40

40 Role of wind shear in squall lines, (), week / 40

41 Clicker question Supercell storms A. have diameters from km B. are usually more violent than MCCs or squall lines, even though they are smaller C. consist of a single extremely powerful cell rather than a collection of smaller cells D. are described by all of the above, (), week / 40

42 , yesterday s news archive surface analysis yesterday s official storm report (), week / 40

43 Clicker question Which of the following statements about tornados is false? A. Strong tornadic winds result from extremely large pressure gradients (100 mb/.1 km, compared to 35 mb/1000 km difference in sea level pressure across North America). B. The wind speed can be measured directly in a tornado ( km/hr, or mph). C. They can span a wide range of sizes and last from minutes to hours. D. They normally travel northeastward at about 50 km/hr (30 mph), more or less., (), week / 40

44 Tornado Safety, Stay inside, in a basement If no basement, go to interior of house. Cover yourself with a mattress. Evacuate a mobile home! If no fixed building exists, lie flat in the ground away from the mobile home. If driving, try to avoid tornado by driving at right angles to path (they move southwest to northeast). If you cannot avoid path while driving, abandon vehicle and seek nearby shelter or run low to the ground away from road. (), week / 40

45 Tornado formation need a mesocyclone (large vortex, example of how to get one shown in figure) contraction of the mesovortex to a smaller vortex increases windspeed by conservation of angular momentum tornado genesis is actually not well understood, (), week / 40

46 NMT on Nova, NOAA safety (), week / 40